It is known that adding polymer to asphalt improves the high temperature performance grade (PG) of paving asphalt cements as defined under the test methods established by the Strategic Highway Research Program (SHRP). Commonly used polymers include ethylene vinyl acetate (EVA) copolymers and styrene-butadiene-styrene triblock (SBS) copolymers. These polymers may be blended into the asphalt using high shear mix conditions to ensure proper dispersion of the polymer. Of the polymers used, SB or SBS polymers are preferred because of their compatibility with a large number of asphalts.
Blending crosslinkable polymers with paving asphalts produces a change in the viscoelastic behavior of the asphalt. The change in behavior can be attributed to the formation of a network structure as well as to an increase in viscosity and elasticity. The change in viscoelastic properties is related to the amount of polymer added. Network formation occurs above some finite polymer concentration. Thus, higher temperature PG grades are achieved as the polymer, e.g., SBS, concentration in the asphalt increases. However, the SBS is more costly than the asphalt, so for economic reasons, its use should be kept to a minimum. The amount of SBS polymer needed can be reduced by adding a vulcanizing (crosslinking) agent such as sulfur. The crosslinking agent allows the network structure to form at lower polymer concentrations. This improves polymer efficiency, reducing the amount of polymer needed to make specific grade of asphalt composition.
U.S. Pat. No. 4,145,322 to Maldonado et al. teaches that the addition of sulfur can improve the efficiency of the SB or SBS, decreasing the amount needed to achieve a desired grade. The sulfur is believed to act as a vulcanizing agent yielding crosslinks between the SB or SBS and asphalt. Typically, with sulfur crosslinking a 3% SBS/asphalt mixture has about the same SHRP high temperature PG grade as an uncrosslinked 5% SBS/asphalt mixture.
U.S. Pat. No. 5,601,697 to Miller et al., incorporated herein by reference, teaches SDA-produced asphalts made by blending SDA bottoms with an aromatic extract. Preferably, the aromatic extract is produced from an extraction process such as that employed in lubricating oil production. In one aspect, the SDA-produced asphalt can contain added polymers which can be vulcanized in situ with the asphalt by using sulfur and accelerators.
One drawback, however, is that polymer-modified asphalt compositions can suffer from a lack of homogeneity over long storage periods, particularly when stored at elevated temperatures. The polymer tends to separates out from the asphalt composition on standing resulting in loss of properties imparted by cross-linking of the polymers.
Accordingly, it would be desirable to not only improve the useful temperature index of an asphalt composition, but improve its storage stability as well.